Variable hydrostatic axial piston machines of swash plate design have a cylinder block and a swash plate whose angle, and thus whose flow of hydraulic fluid, can be varied by means of a servo-piston to which actuating pressure can be applied. Typically, the servo-piston is hydraulically connected to an electrically driven valve. The valve is driven electrically, for which purpose it is customary to use proportional solenoids which are designed to act directly or as so-called nozzle-flapper valves which are designed to act as a pilot control. A version with pilot control is described in U.S. Pat. No. 5,205,201. In the case of variable displacement pumps, this is usually an angle of 0 degrees, whereas in the case of variable displacement motors it is either the maximum or minimum angle.
For special applications, in particular in the case of drive systems with variable displacement motors connected to fast driving vehicles, a good provision is not made in existing equipment to maintain the instantaneous angle in the case of failure of the electronic system or the electronic connection to the adjusting device.
An adjusting device with a stepper motor for a hydrostatic axial piston machine is described in DE 196 08 228 A1. In this known system, a rotary slide valve is operationally connected to a stepper motor. In this known system, the angle is fed back via levers to a rotary sleeve. The disadvantage with this system is that it is necessary to provide a complicated connection, which is free from play to the greatest possible extent, between the swash plate and the control sleeve, as well as to provide a gear between the control slide valve and the stepper motor, since the resolution of the stepper motor is insufficient. This means that the known system is of complicated design and expensive.
Furthermore, U.S. Pat. No. 4,290,447 describes an electrohydraulically proportional valve with an actuator comprising a linear power motor, a valve piston and an axially displaceable valve sleeve. Proportionality between current and force is characteristic of the linear power motor. Providing a spring in the linear motor means that the forces of the motor are in equilibrium with the spring force, as a result of which the force is proportional to the displacement. The tolerances of the springs and the tolerances of the magnetic air gaps are disadvantageous for these known systems. Moreover, the displacement of the linear power motor depends strongly on the friction of the valve piston, on the friction of the fit between the slide valve and the bore, as well as on the pollution in the gap. Furthermore, the magnetic hysteresis of the linear power motor influences the operational performance of such known axial piston machines.
Therefore, it is the object of the invention to create an adjustable hydrostatic axial piston machine in which the adjusting mechanism for the angle is of simple design and operates free from hysteresis to the greatest possible extent in an exactly reproducible fashion over the entire operating range.